1. Field of Invention
The present invention relates to a semiconductor device and a method of fabricating the same, and more generally to a semiconductor device having dual work function metal gates and a method of fabricating the same.
2. Description of Related Art
As the dimension of a semiconductor device is getting smaller, the dimension of the gate structure is accordingly reduced. Thus, to avoid affecting the performance of the device, the thickness of the gate insulation layer has to be reduced accordingly.
Generally speaking, the gate insulation layer includes silicon oxide. A leakage current occurs when the gate insulation layer including silicon oxide becomes thinner.
To reduce the leakage current, one known method is to use a high dielectric constant (high-k) material instead of silicon oxide for forming the gate insulation layer. However, under the condition of using the gate insulation layer including a high-k material, the gate including polysilicon may react with the high-k material to generate a Fermi-level pinning, so that the threshold voltage is increased and the performance of the device is affected.
To avoid an increase in the threshold voltage caused by the reaction between the gate including polysilicon and the high-k material, one known method is to use a metal layer as a gate, and the metal layer is a so-called work function metal layer. However, when the metal layer is used as the gate, the work function of the metal layer is shifted due to the high temperature in the subsequent high-temperature processes, and thus the performance of the device is affected.